Fuel cell devices are electrochemical devices, which enable production of electricity with high duty ratio in an environmentally friendly process. Fuel cell technology is considered to be one of the most promising future energy production methods.
A fuel cell, as presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. The reactants fed to the fuel cell devices undergo a process in which electrical energy and heat are produced as a result of an exothermal reaction. For example in solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with the used fuel 108 producing water and, for example, carbon dioxide (CO2). Between the anode and cathode is an external electric circuit 111 as a load for fuel cell for transferring electrons e- to the cathode. External electric circuit includes a load 110.
In FIG. 2, a SOFC device is presented as an example of a fuel cell device which can utilize as fuel, for example, natural gas, bio gas, methanol or other compounds containing hydrocarbons. The SOFC device in FIG. 2 includes planar-like fuel cells in stack formation 103 (SOFC stack). Each fuel cell includes anode 100 and cathode 102 structure as presented in FIG. 1. Part of the used fuel is recirculated in feedback arrangement through each anode. The SOFC device in FIG. 2 includes a fuel heat exchanger 105 and a reformer 107. Heat exchangers are used for controlling thermal conditions in a fuel cell process and there can be located more than one of them in different locations of a SOFC device. The extra thermal energy in circulating gas is recovered in the heat exchanger 105 to be utilized in a SOFC device or outside in a heat recovering unit. The heat recovering heat exchanger can thus be located in different locations as presented in FIG. 2. Reformer 107 is a device that converts the fuel such as, for example, natural gas to a composition suitable for fuel cells, for example, to a composition containing half hydrogen and other half methane, carbon dioxide and inert gases. The reformer is not, however, necessary in all fuel cell implementations, but untreated fuel may also be fed directly to the fuel cells 103.
By using measurement device 115 (such as fuel flow meter, current meter and temperature meter) measurements are carried out for the operation of the SOFC device from the through anode recirculating gas. Only part of the fuel used at anodes 100 (FIG. 1) of the fuel cells 103 is recirculated through anodes in feedback arrangement 109 and thus in FIG. 2 is presented diagrammatically also as the other part of the gas is exhausted 114 from the anodes 100.
In fuel cell systems, fuel cell stacks can be grouped to serially and/or parallel connected groups. Voltage levels arising among fuel cell stacks are isolated between fuel cell stacks to avoid unwanted current loops. Exemplary voltage levels can be isolated also from installation structure of the fuel cell device, which includes for example supporting structures, and piping, which includes for example fuel feed-in lines.
Adequate and stable electronic isolation can be difficult to achieve in the case of demanding and chemically aggressive environments, such as is the case with, for example, high temperature fuel cell applications. In these contexts corrosion, thermomechanical stress, material degradation or electrochemical phenomenon, which may be caused by and/or accelerated by voltage differences of fuel cells, may each as individual phenomenon and together cause conductive routes through the isolation and/or cause isolation breakdowns. Internally arising leakage currents among fuel cells can degrade load power and may cause irreversible degradation to fuel cells, and even completely break the fuel cell stacks, which those leakage currents have had a possibility to heavily influence. These effects can be partially or totally avoided so that the galvanic isolation levels are continuously monitored and a lack of isolation is found early enough to leave time for desired protective actions.
Deterioration of the isolation may not be easily detected in voltage and/or current measurements outside of the fuel cell device. If the fuel cell stacks are electrically unfloating, fault current measurement can be used to monitor leakage currents. However, this method is not proper to identify locations of leakage currents, and is not proper to observe leakage currents inside the fuel cell device.